The present invention relates to a photometric apparatus for a camera, and more particularly, to a photometric apparatus for use with a camera capable of logarithmetically converting a photocurrent flowing through a photoelectromotive element to produce an output of photometric data simultaneously with the initiation of a photometric operation.
It is known to employ a circuit arrangement shown in FIG. 1 of a photometric apparatus for a camera in which a photocurrent generated in a photoelectromotive element for photometry such as a silicon photodiode or a solar battery in accordance with the brightness of an object being photographed is picked up as a logarithmically converted voltage. Specifically, with the photometric apparatus shown in FIG. 1, a photoelectromotive element 3 is connected between a non-inverting input terminal .sym. of an operational amplifier 1 which is connected to an output terminal of a reference voltage circuit 2 and an inverting input terminal .crclbar. of the amplifier 1, and a logarithmic compression diode 4 is connected between the inverting input terminal .crclbar. and an output terminal of the amplifier 1. Power supply terminals for the amplifier 1 and the reference voltage circuit 2 are connected through a power switch 5 which is closed by a shutter release operation to a battery 6. A resistor 7 provides a bias current for activating the amplifier 1. In the photometric apparatus in which the photoelectromotive element 3 is connected between input terminals .sym. and .crclbar. of the amplifier 1, the same voltage is always maintained between both terminals of the photoelectromotive element 3 and hence good linearity of photosensitive characteristics may be obtained.
However, the photometric apparatus requires a considerable time period until an output of a normal photometric datum is derived from the operational amplifier 1 after the steady state is attained upon closing of the power switch 5 by a shutter release operation. The main reason is that there are the junction capacity on photo-electromotive element 3 and diode 4 and the distributed capacity on other circuit portions so that it requires time to discharge the charges across these electrostatic capacities by a photocurrent of photoelectromotive element 3. Specifically, an output of the amplifier 1, as shown in FIG. 2 with a solid line, swings up to a level of a power voltage Vcc (or the ground potential) at the moment power switch 5 is closed and subsequently the output tends toward the photometric voltage V.sub.BV corresponding to the brightness of an object being photographed as the charge across the electrostatic capacities are discharged by the photocurrent. The photometric voltage V.sub.BV is brought into a steady state when the zero bias condition is obtained between both terminals of the element 3 as the charges are discharged. As a result, under a condition that an object being photographed is so dark that the photocurrent is as low as 10.sup.-11 to 10.sup.-12 amperes, the time period from the closed time of a power switch to the time when an output of the amplifier 1 is brought into a steady state of the normal photometric voltage V.sub.BV requires several seconds to tens seconds. To this end, an improved photometric apparatus has been devised in which a second diode 8 is connected in a reversely parallel relationship with the above-stated logarithmic compression diode 4. In the photometric apparatus, as shown in FIG. 2 with a dotted line, although a photometric voltage is brought into a steady state slightly sooner compared with an apparatus without diode 8, one cannot expect a sharp reduction of time. Additionally, a number of countermeasures have been proposed as means for compensating the time lag of response by the above-stated capacity components. However, all these conventional countermeasures require a delicate high impedance circuit, a complicated circuit construction of IC, an element for applying a photobias or the like, resulting in an inevitably complicated construction of a photometric apparatus. Furthermore, even with such countermeasures a time lag over several tens msec is unavoidable.
On the other hand, when a high speed continuous photography is conducted with an apparatus such as a motor drive, the time period from the initiation of a shutter release operation to the completion thereof is about 5 msec at the most. Accordingly, with an apparatus in which power switch 5 is closed in response to a shutter release operation, it is disadvantageously impossible to take a picture of a proper exposure with a motor drive in an areas of comparatively dark surroundings.
However, a power source for a photometric apparatus is restricted to the comparatively small capacity source such as a button shaped silver battery or a mercury battery and hence it is requested to keep a consumed power minimum. Accordingly, if a power switch is closed prior to a shutter release operation so that a photometric apparatus may be brought into an operational condition, the time lag of response by the above-stated capacity components may be improved while the power consumption is disadvantageously high. Additionally, as the power switch is kept off to save the power while not in use, a photography may be conducted in two operational steps and there is the possibility of missing a shutter chance to a rapidly moving object being photographed.